Page 331 - Concise Pathology for Exam Preparation ( PDFDrive )

316 SECTION II Diseases of Organ Systems

Causes of Secondary or Acquired Aplastic Anaemia

• Infections: Hepatitis viruses, EBV, human immunodeficiency virus (HIV), parvovirus
and mycobacteria
• Radiation and chemicals: Benzene, lindane (gamma benzene hexachloride) and DDT
• Drugs: Drugs can produce aplastic anaemia either due to direct toxic effect (dose-
dependent and predictable response) or idiosyncratic reactions (dose-independent
and unpredictable response). The following drugs are implicated in aplastic
anaemia:
• Cytotoxic (alkylating agents and antimetabolites)
• Antibacterial (chloramphenicol, sulfonamides, isoniazid and arsenicals)
• Antirheumatic (oxyphenbutazone, phenylbutazone, indomethacin, gold salts and
D-penicillamine)
• Antidiabetic (tolbutamide and chlorpropamide)
• Miscellaneous (chlorothiazide, mepacrine, hydralazine, acetazolamide, potassium
perchlorate, carbamazepine and carbimazole)
• Miscellaneous causes: Pancreatitis, PNH and eosinophilic fasciitis

Pathogenesis

Haematopoietic failure may be due to various mechanisms, eg, decreased number of stem
cells in the marrow, defective stem cells or a defective microenvironment that fails to
sustain normal haematopoiesis.

Clinical Features
• Petechiae, ecchymoses, nasal and GIT bleeding due to thrombocytopenia
• Infections due to neutropenia
• Weakness, easy fatigability, pallor and breathlessness due to anaemia

Laboratory Diagnosis
Aplastic anaemia is diagnosed if any two of the following are present:
• Hb # 10 g/dL
• Neutrophil count #1500/mm 3
• Platelet count #50,000/mm 3

Peripheral Smear
• Shows a normocytic-normochromic anaemia, leucopenia (neutropenia with relative
lymphocytosis) and thrombocytopenia
• Mild macrocytosis is occasionally seen.
• Corrected reticulocyte count is low.
• May be differentiated from infiltrative causes of pancytopenia based on the absence of
teardrop poikilocytes and a leukoerythroblastic picture, both of which suggest an infiltra-
tive process. The presence of dyserythropoietic cells and hypogranulated neutrophils
indicates myelodysplasia and differentiates aplastic anemia from dysplastic causes of
pancytopenia.

Bone Marrow
Dry tap; markedly hypocellular or acellular marrow with increased iron stores
Grading of Aplastic Anaemia
Aplasia is said to be ‘severe’ if any two of the following are present:
3
1. Neutrophil count is less than 500/mm .
3
2. Platelet count is less than 20,000/mm .
3
3. Absolute reticulocyte count ,40,000/mm and marrow biopsy showing ,25% of
normal cellularity, or 25–50% marrow cellularity with ,30% haematopoietic cells.
3
Criteria for ‘very severe’ aplasia are similar, except granulocyte count # 200/mm .
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